As announced by Mozillazine.org [mozillazine.org], Firefox 0.10, also known as Firefox 1.0PR, has been updated to version 0.10.1 in order to patch a security hole discovered last week.
This is a pre-emptive patch; the security hole is not known to have been exploited yet. As stated by Mozilla.org, "This security update is also another example of the Mozilla Foundation identifying and fixing security vulnerabilities before they are exploited by malicious hackers."
Users of Firefox 1.0PR (version 0.10) can wait for the built-in automatic update check or force an update using the Firefox toolbar function Tools->Options->Advanced->Software Update->Check Now. This downloads a very small XPI to install the upgrade, and avoids having to download the entire 0.10.1 browser. Click to install the update, then exit and restart Firefox.